Exhaust duct having modular, multi zone, spirally arrayed cooling coils and method for cooling

ABSTRACT

An exhaust duct for cooling an exhaust flue. One or more modular coolant coils are disposed about the outer circumference of or within the flue interior cavity for thermal communication with exhaust gas. Each coil has a helical spiral profile extending along the flue axial dimension, an interior lumen there through for passage of coolant provided by a cooling system, and a respective inlet and outlet for respective intake and discharge of coolant. Optionally at least one remotely adjustable valve is coupled to the coolant coil and an industrial automation controller, for regulation of coolant flow rate within the coil. The coolant coils may incorporate one or more coolant temperature sensors in communication with the controller. A plurality of exhaust duct cooling coils may be under common control of the controller, for allocation of coolant among the coils and other portions of the cooling system.

BACKGROUND OF THE DISCLOSURE

1. Field of the Invention

The invention relates to fluid-cooled exhaust flues or ducts for transferring heat from an exhaust gas to a second cooling fluid flowing in cooling conduits that are in thermal communication with the duct. The present invention is suited for application to combustion or process exhaust flues, such as are found in electric arc steel mill furnaces, petroleum or other chemical refining plants or electric power generation plants.

2. Description of the Prior Art

Exhaust flues, such as found in electric arc furnaces in steel mills, require reduction in exhaust gas temperature before the exhaust is released to the atmosphere, for conformity with environmental regulations or to reduce damage to exhaust flues that may result from prolonged exposure to high temperatures. In some applications heat extracted from the exhaust gas is used for cogenerative power generation or other thermal energy needs, including building or factory process heating.

Traditionally, exhaust flues or ducts have been cooled with fluids, such as treated water, flowing through cooling conduits in thermal communication with the flue. Water passing through the cooling conduit absorbs heat from the flue exhaust, is re-cooled to a lower temperature by an air cooling tower or other heat exchanger, and recycled through a continuous loop to the exhaust flue.

United States Patent Application Publication No. US 2006/0291523 shows arrays of axially-oriented cooling tubes or channels about the periphery of an exhaust duct, wherein the cooling fluid is pumped parallel to the exhaust gas. Proximally adjoining cooling tubes are welded together along their axial lengths to form a unitary circumference of the cooling duct flue. Adjoining tube fluid carrying interiors are interconnected by elbow bends at each end of the exhaust flue, forming a serpentine, undulating cooling fluid flow path. In other embodiments the cooling fluid flows in U- or C-shaped channels formed about the outer periphery of the duct. As one skilled in the art can appreciate, such tight elbow bends between proximal adjoined cooling tubes creates relatively higher cooling fluid flow resistance than an equivalent length of straight tube. The higher fluid flow resistance must be overcome by use of higher power consuming cooling flow pumps.

The axially oriented cooling tubes of the US 2006/0291523 publication will also require relatively high cooling water flow rates in order to avoid overheating cooling water proximal the exhaust flue inlet region. More heat must be transferred out of the exhaust flue near its inlet than near its outlet, because the exhaust gas cools as it flows through the flue. If an operator wishes to follow a common cooling practice to maintain the cooling water below its boiling point one must maintain a relatively high flow rate through the axially oriented tubes so that the cooling water does not overheat proximal the exhaust flue inlet. Given the tube orientation, cooling water heated proximal the flue intake in a cooling tube flowing toward the exhaust must travel a circuitous path along the entire duct length and back before it is exhausted to an outlet manifold. During such a circuitous path the heated fluid has limited remaining capacity to absorb heat from flue at the downstream end. The long, circuitous flow path in turn increases cooling water pumping power requirements, in addition to the higher pumping requirements attributed to higher flow rate and need to overcome pumping resistance in tight elbow bends.

U.S. Pat. No. 4,556,104 references a heat exchanger for heating especially an organic liquid transfer fluid by way of combustion gasses from a burner of fossil fuel. It states that a flue or cooling conduits proximal the inlet hot combustion gasses can be shielded with a refractory material or by spirally winding a single continuous loop of cooling coil about the flue interior at varying winding pitch rates, with closer winding near the flue intake and wider winding proximal the flue exhaust. While in theory tighter coil winding proximal the flue intake would enable a greater rate of heat transfer, the disclosure appears to be in the context of intentionally heating the fluid in the cooling coil. Logically if fluid in the cooling deviates from a desired temperature all one would do would be to adjust the heater exhaust temperature up or down to achieve the desired fluid temperature. This is not possible in the context of a steel mill, power generation plant or other industrial process application, where the exhaust temperature of the flue gas cannot be adjusted without compromising process efficiency or quality.

It is also noted that the continuous cooling coil shown in the U.S. Pat. No. 4,556,104 must be replaced in toto, or a section of which must be replaced in situ in case of cooling coil leak or other failure.

Thus, a need exists in the art for a an exhaust flue gas cooling duct that selectively varies cooling fluid circulation rate in different zones of the duct, so that for example, more heat can be transferred away from the duct proximal the relatively hotter duct intake region and a lower circulation rate can be utilized in the relatively cooler duct exhaust region, thereby conserving fluid flow capacity and cooling pumping power requirements.

Another need exists for an exhaust cooling duct cooling coil geometry that reduces fluid pumping resistance than required for previously known axially oriented cooling coils with relatively tight elbow radius between adjoining axial coil sections.

Yet another need exists for an exhaust cooling duct having modular cooling coils that can be field installed and repaired with relatively lower effort than known integrated, single coil cooling systems, preferably without disrupting adjoining associated cooling system structure, manifolds and valving.

SUMMARY OF THE INVENTION

Accordingly, an object of the invention is to create a duct cooling system that enables selective variation of duct cooling parameters in separate cooling zones, so that cooling fluid water usage can be optimized and cooling water pumping power can be reduced.

Another object of the present invention is to create a duct cooling system that reduces cooling coil pumping resistance.

Yet another object of the present invention is to create a duct cooling system employing modular multi-zone cooling subsystems that can be selectively repaired or replaced without the need to disrupt other unaffected duct cooling subsystems and related components.

These and other objects are achieved in accordance with the present invention by the duct cooling system of the present invention which employs modular, multi-zone spirally oriented cooling conduits.

One aspect of the present invention is an exhaust duct cooling system having an exhaust flue defining a interior cavity for passage of exhaust gas there through along an axial dimension thereof. A coolant coil is disposed about the flue external circumference or interior cavity for thermal communication with exhaust gas. The coolant coil has a helical profile extending along the flue axial dimension, an interior lumen there through for passage of coolant, and a respective inlet and outlet for respective intake and discharge of coolant. Optionally, at least one flow regulator, which is preferably but is not required to be an adjustable valve, is coupled to the coolant coil, for regulation of coolant flow rate within the coil. Optionally the adjustable valve may be remotely controlled, such as by a controller of an industrial automation system.

Another aspect of the present invention is directed to an exhaust duct cooling system, having an exhaust flue defining an interior cavity for passage of exhaust gas there through along an axial dimension thereof. A plurality of coolant coils are disposed serially about the flue external circumference or within the flue interior cavity for thermal communication with exhaust gas. Each respective coil has a helical profile extending along the flue axial dimension, an interior lumen there through for passage of coolant, and a respective inlet and outlet for respective intake and discharge of coolant. Optionally, at least one adjustable valve is coupled to each respective coolant coil, for regulation of coolant flow rate within the coil. This aspect of the invention optionally may also feature an intake manifold in common parallel fluid communication with the inlets and an exhaust manifold in common parallel fluid communication with the outlets.

Yet another aspect of the present invention is directed to a method for cooling an exhaust duct cooling system having an exhaust flue that defines an interior cavity for passage of exhaust gas there through along an axial dimension thereof. The method includes orienting at least one coolant coil about the flue external circumference or within the flue interior cavity for thermal communication with exhaust gas. The coil has a helical profile extending along the flue axial dimension, an interior lumen there through for passage of coolant, a respective inlet and outlet for respective intake and discharge of coolant, and a flow regulator that is optionally at least one adjustable valve coupled to the coolant coil. As an additional option the adjustable valve may be remotely controlled by a controller of an industrial automation system coupled thereto, for regulation of coolant flow rate within the coil. The method includes feeding coolant through the intake and discharging the cooling through the outlet at a flow rate; measuring coolant temperature at least the outlet with a temperature sensor and regulating coolant flow rate with the flow regulator. Optionally the temperature sensor is remotely coupled to the controller; the controller regulating coolant flow rate with the adjustable valve in order to achieve a desired outlet coolant temperature. Optionally a plurality of coolant coils, associated valves and temperature sensors may be in communication with the industrial automation controller, so that the controller can optimize coolant utilization within the aggregate cooling system.

One or more of the objects, aspects and features of the present invention may be selectively employed jointly in combinations or severally by one skilled in the art when practicing the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view showing application of the present invention in an exemplary steel mill;

FIG. 2 is a plan view of an embodiment of an exhaust duct of the present invention;

FIG. 3 is an elevational sectional view of the exhaust duct of the present invention taken along 3-3 of FIG. 2;

FIG. 4 is a detailed elevational sectional, view of the exhaust duct of the present invention taken along 4-4 of FIG. 2;

FIG. 5 is a schematic view of a plurality of exhaust ducts of the present invention with optional remote control valves thereof operated by a controller of the present invention within an industrial automation communication and control system;

FIG. 6 is a detailed elevational sectional view similar to that of FIG. 4, of another embodiment of the exhaust duct of the present invention;

FIG. 7 is a schematic view similar to that of FIG. 5, of the embodiment of FIG. 6; and

FIG. 8 is a schematic view similar to that of FIG. 1, showing application of an alternative embodiment of the present invention in an exemplary steel mill.

To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.

DETAILED DESCRIPTION

After considering the following description, those skilled in the art will clearly realize that the teachings of the present invention can be readily utilized in exhaust cooling ducts for different industrial applications.

General Cooling System Overview

FIG. 1 shows schematically a steel mill 10 including an electric arc furnace 15 with an exhaust flue 20. The exhaust, arrow F, is routed to a particulate drop out box 25, and thereafter to a forced draft cooler 30 where finer particulates are extracted from the exhaust. The plant 10 has a main cooling supply header 40 and a main return header 45 that is responsible for routing coolant in a defined portion, of the plant 10 that may include the entire plant. Coolant is recycled from the return header 45 to a coolant recycling heat exchanger 50 (here shown as an exemplary air cooling tower) that lowers the return coolant temperature so that it may be returned to the plant via the supply header 40. The exhaust flue 20 includes a plurality of exhaust ducts 100 of the present invention. Here in the exemplary embodiment of FIG. 1 there are shown five such separate exhaust duct 100 sections, though it should be understood by those skilled in the art that the number and sizing of such ducts will be specific application dependent.

The exemplary embodiment of the present invention is shown herein in a steel mill application. However, as previously stated, it should be understood by those skilled in the art that the present invention may be applied to other exhaust flue environments, such as by way of nonlimiting example in power plants or chemical processing plants.

As a practical matter any industrial plant utilizing cooling water or other cooling fluid does not have an infinite supply of coolant, thereby necessitating recycling of coolant after subsequent cooling. Therefore coolant must be monitored and allocated within the plant often in accordance with dynamically changing cooling needs. Ideally coolant should not be wasted, overheated to a mixed fluid-gas thermodynamic phase, cooling dwell time necessary to reduce coolant temperature for recycling back to the plant should be minimized and coolant pumping should be minimized in order to reduce plant operating costs.

Exhaust Duct and Cooling System Structure

FIG. 2 shows a cooling duct assembly 100 of the present invention, including flue 110. The flue 110 is of known construction, having a cylindrical or other desired cross-sectional profile, with intake flue flange 112 and exhaust flue flange 114. Exhaust flows through the flue 110 in the direction of the double arrow F. The flue may be constructed of any known material for the application, including exemplary rolled sheet steel, and may include an insulative lining of refractory material or ceramic in any portion thereof.

The exhaust duct 100 includes at least one and preferably a plurality of spiral wrapped coolant coils 120 about the flue 110. In a preferred, but not required embodiment, the individual serial coils form separate circuit zones (C1, C2, C3 . . . C(N−1), CN), and may have a varying number of winding turns and winding pitch as selected by the designer. As one skilled in the art can appreciate, the heat absorption capacity (and conversely flue cooling capacity) of any individual cooling coil 120 is a function of the number of windings, their pitch, coil tubing material, tubing diameter, thermal capacitance properties of the coolant and coolant flow rate, among others. The coils 120 may be wrapped about the exterior circumference of the flue 110 and in other applications about the interior of the flue.

Each respective coil 120 may have any desired cross-section and be constructed of any known material suitable for exhaust flue cooling applications. An exemplary cross-section and material for coolant coils shown in the figures herein may be round steel tubing that can be readily formed into a helical spiral shape. The relatively gentle spiral bends of larger winding diameter have lower fluid flow resistance than the relatively tighter radius 180 degree sharp elbow bends and long tube runs required at the ends of previously known axially oriented cooling tube constructions, thereby reducing pumping power needed to pump coolant through the cooling tube coils 120.

Exemplary dimensions for coolant coils of the present invention as applied in steel mill exhaust flues are:

helical winding profile inside diameter of 63-87 inches (1.6-2.2 meters), preferably constructed of 2 inch (50 mm) or 2.5 inch diameter (64 mm) schedule 80 or schedule 160 pipe; or 3 inch (76 mm) schedule 40 or schedule 30 pipe;

any desired helical profile axial length, but often 17-20 feet (5.2-6.2 meters);

2-N (often 5-9) zone coil circuits within the helical profile; and

each coil circuit having an internal surface area of 43-76 square feet (3.9-6.9 square meters).

The coils 120 are of modular construction and individually replaceable or serviced in the field after exhaust duct construction without interaction or disabling of other neighboring coils. Referring generally to FIGS. 2-4, each modular coil 120 has a cooling tube inlet 122 and a cooling tube outlet 124. Cooling tube caps 126 seal the respective ends of each coil 120, for maintaining coolant retention integrity without leaks. For additional exhaust duct cooling system modularity, in each cooling zone, C1-CN, the cooling coil tubes preferably are coupled in parallel to a common coolant supply header 130 that has a coolant supply inlet 132 connected to the cooling system coolant supply and a coolant supply manifold 134. Each cooling zone preferably includes a manual shut off valve 136 having an inlet coupled to the coolant, supply manifold 134 and having an outlet coupled to the cooling tube inlet 122 by way of a reinforced flexible hose 138, shown schematically in the figures. Completing the cooling flow circuit in each cooling tube 122, the cooling tube outlet 124 is coupled in parallel to cooling water exhaust manifold 140, having a cooling water exhaust outlet 142 that is connected to the cooling system return loop to the cooling tower, for cooling of the coolant and eventually recycling to the cooling system supply.

In the preferred embodiment shown the exhaust manifold 140 is coupled in parallel to all of the cooling tube outlets 124 in an associated set of zones by each respective reinforced flexible hose 148, shown schematically in the figures and in turn to cooling water exhaust manual shut-off valve 146.

Any cooling tube 122 can be included or isolated from the cooling system by actuation of the respective intake and exhaust manual shut-off valves 136, 146, for removal and replacement or servicing, without impacting other zones or the respective supply or exhaust manifold structures 134, 144. The manifolds 134, 144 are coupled to the exhaust duct 100 by header supports 150.

Coolant Flow Control

Referring to FIG. 5, optional coolant flow regulation and/or calibration in each zone C1-CN is preferably accomplished by actuation of flow control valve 160. In alternative configurations of the present invention coolant flow regulation can be accomplished by other flow regulator structure, including initial selection of cooling tube 122 diameter, or by adjustable regulator structure, including by way of non-limiting example a flow restrictor plate, venturi or orifice in series with the cooling tube or the manual shut-off valves 136 or 146. In order to achieve the benefits of optional automated plant process control afforded by modern industrial control systems each of the flow control valves 160 is preferably remotely actuated by an industrial automation controller 180 via a communications pathway, depicted schematically as 182. A controller may be implemented by way of example through a programmable logic controller (PLC) or a general purpose digital computer emulating a PLC, also known as a “soft PLC” having a processor that executes stored process control software commands via a software operating system. The controller 180 and valve 160 communication pathway may be implemented by any way known in the industrial automation and control field, including by way of example hard wired twisted cable pair, shielded coaxial cable, computer communications bus, Internet, Intranet, or wireless remote communication.

Advantageously the controller 180 also monitors temperature in each coolant coil 120 by way of a temperature sensor, such as outlet temperature sensor 170 via communications pathway 182. An inlet temperature sensor 172 may also be employed. The controller 180 preferably adjusts coolant flow rate via valve 160 in each coolant coil based in at least part by the temperature measurements obtained from temperature sensors 170, 172 or a combination of measurements from both, such as via a known temperature control feedback loop. The controller may also utilize other plant operational information in regulating coolant flow rates in each cooling coil 120. For example, as shown in FIG. 5, if another part of the plant requires higher priority allocation of coolant, permissible maximum temperatures in one or more of the regulated cooling zones C1-CN may be raised to free up coolant capacity in higher priority zones.

Preferred multiple zone C1-CN construction of the duct system of the present invention enables more precise heat transfer with overall lower coolant pumping effort than known designs that incorporate axially oriented parallel tube cooling. For example, known axial oriented cooling tube constructions require long pumping pathways through a serpentine tube layout, thereby generating more coolant pumping resistance than the relatively shorter, large diameter helical windings of the individual zone cooling tubes 120 of the present invention.

In practice of the present invention, an exhaust duct 100 may have a single cooling zone C that is coupled to a common plant cooling system with other exhausts ducts individually having one or more separate serial coil 120 cooling zones CN. Each “zone”, whether jointly or severally part of a single exhaust duct 100 assembly or a consolidation of zones in multiple exhaust duct assemblies, may be controlled separately or as part of an aggregate combination or sub combination by an industrial plant coolant control system.

The present invention cooling system enables precise fine tuning of flow rates in each zone C1-CN. For example in FIG. 2, zone C1 is closest to the exhaust intake of the duct section 100. The flow rate through the zone C1 tube 120 may be established by its associated flow control valve 160 (or if the flow control valve 160 is not used, by adjustment of the manual valves 136 or 146, or by adjustment or replacement of other flow regulation structure, including remote actuated valves actuated by solenoids), so that cooling water measured by outlet temperature sensor 170 does not exceed a desired maximum temperature: for example 140 degrees Fahrenheit (60 degrees Celsius). Exhaust gas cools as it traverses flue 110 from zone C1 to zone C2. Assuming the coolant coil for zone C1 is constructed the same as that of zone C1, less coolant flow should be required to remain within the maximum temperature setpoint and so on as the exhaust traverses the flue. By maximizing flow rate efficiency for each zone below the maximum temperature, aggregate coolant pumping for all zones is reduced, freeing up coolant for other applications within the plant as well as reducing pumping costs.

As an additional option to conserve coolant and provide additional coolant allocation flexibility within an industrial plant, the alternative embodiment of the present invention shown in FIGS. 6 and 7 provide for coolant recirculation within any one or more of the cooling zones within a single exhaust duct assembly or in a plurality of exhaust duct assemblies within a plant. Coolant bypass 190 includes a remote actuation valve 192 of known design, actuated by a bypass valve solenoid control 194 in communication with PLC 180. In this exemplary embodiment the manual shut-off valves 136, 146 for supply and exhaust shown in the embodiments of the prior figures are also replaced by respective remote actuated supply valve 196, actuated by supply valve solenoid 198, and exhaust valve 200, actuated by exhaust valve solenoid 202. As one skilled in the art can appreciate, any of the embodiments of the present invention can substitute different flow restrictors than those shown in the figures. For example a solenoid controlled remote actuated valve may be substituted for the flow control valve 160 in the embodiment of FIG. 2 and conversely, flow control valves can be substituted for one or more of the solenoid controlled remote actuated valves 192, 196, and 200 in the embodiment of FIG. 6.

in normal operation of the embodiment of FIGS. 6 and 7, bypass valve 192 is closed, and coolant circulation is from the supply manifold 130 with return to the exhaust manifold 140. When coolant recirculation is desired, for example when fresh coolant must be allocated to another portion of the plant cooling system, supply valve 196 may be closed in part or totally to reduce fresh coolant supply from the supply manifold 130 to the cooling tube 120. The bypass valve 192 is opened in part or totally, in order to cause coolant recirculation within cooling tube 120. Exhaust valve 200 also is closed in part or totally to reduce flow of recirculated coolant to the exhaust manifold 140. Coolant recirculation may be modified based on coolant temperature feedback from the temperature sensor 170 or in response to remote commands from the PLC. For example, if a measured temperature by temperature sensor 170 exceeds a maximum threshold the percentage of recirculated coolant may be reduced by opening further the coolant supply and exhaust valves 196, 200 and/or closing further the bypass valve 192, or any combination of the above.

An optional advantage of the present invention is that the concept of a coolant bypass 190 with bypass valve 192 may be incorporated into an exhaust duct assembly 100 having a single cooling loop 120, so that a user may install a reduced cost cooling duct with a single zone. As shown in FIG. 8, this single zone per duct inventive coolant bypass concept may be incorporated into multiple cooling duct assemblies 100, in which case each separate duct assembly 100 constitutes a “zone” for coolant control purposes, and may be commonly controlled in any combination by the plant coolant flow regulation system.

Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings. 

1. An exhaust duct cooling system, comprising: an exhaust flue defining a interior cavity for passage of exhaust gas there through along an axial dimension thereof; a coolant coil disposed about the flue for thermal communication with exhaust gas, the coil having a helical profile extending along the flue axial dimension, an interior lumen there through for passage of coolant, and a respective inlet and outlet for respective intake and discharge of coolant; and at least one flow regulator coupled to the coolant coil, for regulation of coolant flow rate within the coil.
 2. The system of claim 1, wherein the flow regulator is selected from the group consisting of coolant coil diameter, flow restrictor plate, venturi, orifice, manual or remotely adjustable valve, remote controlled flow valve and solenoid actuated remote control valve.
 3. The system of claim 1, further comprising a temperature sensor coupled to the coil, wherein the temperature sensor is in communication with a control system controller, the flow regulator is a remotely adjustable valve coupled to and remotely actuated by the control system, and the control system regulates coolant flow rate based at least in part on coolant temperature information obtained from the temperature sensor.
 4. The system of claim 3, further comprising respective inlet and outlet temperature sensors coupled to the coil inlet and outlet, for measuring coolant temperature at each location and the control system regulates coolant flow rate based at least in part on coolant temperature information obtained from both of the inlet and outlet temperature sensors.
 5. The system of claim 1, further comprising a plurality of coolant coils disposed about the flue selectively along its length for thermal communication with exhaust gas, each coil having a helical profile extending along the flue axial dimension, an interior lumen there through for passage of coolant, and a respective inlet and outlet for respective intake and discharge of coolant.
 6. The system of claim 5, further comprising a temperature sensor coupled to each respective coil, for measuring coolant temperature.
 7. The system of claim 6, wherein the temperature sensors are in communication with a control system controller, and at least one adjustable valve is coupled to at least one of the cooling coils and is remotely actuated by the control system, and the control system regulates coolant flow rate in each respective coil based at least in part on coolant temperature information obtained from its respective temperature sensor.
 8. The system of claim 7, further comprising respective inlet and outlet temperature sensors coupled to at least one of the coil's inlet and outlet, for measuring coolant temperature at each location and the control system regulates coolant flow rate based at least in part on coolant temperature information obtained from both of the inlet and outlet temperature sensors.
 9. The system of claim 5, further comprising respective inlet and outlet temperature sensors coupled to each respective coil inlet and outlet, for measuring coolant temperature.
 10. The system of claim 9, wherein the respective temperature sensors for each respective coil are in communication with a control system, each respective coil also having a respective adjustable valve flow regulator coupled to and remotely actuated by the control system, and the control system regulates coolant flow rate in each respective coil based at least in part on difference in measured coolant temperature between the inlet and outlet.
 11. The system of claim 1, further comprising a coolant flow bypass between the coolant coil inlet and outlet, for selectively recirculating coolant.
 12. An exhaust duct cooling system, comprising: an exhaust flue defining a interior cavity for passage of exhaust gas there through along an axial dimension thereof; a plurality of coolant coils disposed serially about the flue for thermal communication with exhaust gas, each coil having a helical profile extending along the flue axial dimension, an interior lumen there through for passage of coolant, and a respective inlet and outlet for respective intake and discharge of coolant; an intake manifold in common parallel fluid communication with the inlets; and an exhaust manifold in common parallel fluid communication with the outlets.
 13. The system of claim 12, further comprising at least one flow regulator coupled to each respective coolant coil, for regulation of coolant flow rate within the coil, the flow regulator selected from the group consisting of coolant coil diameter, flow restrictor plate, venturi, orifice, manual or remotely adjustable valve, remote controlled flow valve and solenoid actuated remote control valve.
 14. The system of claim 13, wherein at least one flow regulator is a remotely adjustable valve that is adjustable by a control system controller, and wherein the at least one of the adjustable valves is coupled to and remotely actuated by the control system.
 15. The system of claim 14, further comprising a temperature sensor coupled to each respective coil, for measuring coolant temperature, wherein the temperature sensors are in communication with a control system controller, each respective adjustable valve is coupled to and remotely actuated by the control system, and the control system regulates coolant flow rate in each respective coil based at least in part on coolant temperature information obtained from its respective temperature sensor.
 16. The system of claim 15, further comprising respective inlet and outlet temperature sensors coupled to at least one of the coil's inlet and outlet, for measuring coolant temperature at each location and the control system regulates coolant flow rate based at least in part on coolant temperature information obtained from both of the inlet and outlet temperature sensors.
 17. The system of claim 12, wherein the coil inlets and outlets are coupled to the respective intake and exhaust manifolds by serviceable fluid fittings.
 18. The system of claim 12, further comprising a coolant flow bypass between at least one coolant coil inlet and outlet, for selectively recirculating coolant.
 19. The system of claim 18 further comprising a remote actuated valve in the bypass.
 20. A method for cooling an exhaust flue that defines an interior cavity for passage of exhaust gas there through along an axial dimension thereof, comprising: orienting a coolant coil about the flue for thermal communication with exhaust gas, the coil having: a helical profile extending along the flue axial dimension, an interior lumen there through for passage of coolant, a respective inlet and outlet for respective intake and discharge of coolant, and coupling at least one flow regulator to the coolant coil, for regulation of coolant flow rate within the coil; feeding coolant through the intake and discharging the coolant through the outlet at a flow rate; measuring coolant temperature in the coil with at least one temperature sensor coupled thereto; and regulating coolant flow rate with the flow regulator based at least part on measured coolant temperature.
 21. The method of claim 20, wherein the flow regulator is a valve, the temperature sensor and valve are coupled to a controller in communication therewith, the controller performing the measuring and regulating steps.
 22. The method of claim 21, wherein the coolant is provided by a coolant system coupled to the inlet and outlet of the coil and the controller varies the desired coolant temperature based on coolant system operational parameters.
 23. The method of claim 20, further comprising: orienting a plurality of coolant coils disposed serially about the flue for thermal communication with exhaust gas, each coil, having a helical profile extending along the flue axial dimension, an interior lumen there through for passage of coolant, and a respective inlet and outlet for respective intake and discharge of coolant; coupling at least one adjustable valve flow regulator to each coolant coil, for regulation of coolant flow rate within the respective coil; and for each respective coil, performing the feeding, measuring and regulating steps.
 24. The method of claim 23, wherein the respective temperature sensors and valve for each respective coil are coupled to a controller in communication therewith, the controller performing the measuring and regulating steps for each respective coil.
 25. The method of claim 24, wherein the coolant is provided by a coolant system coupled to the respective inlet and outlet of each respective coil and the controller varies the desired coolant temperature in each respective coil based on coolant system operational parameters.
 26. The method of claim 23, comprising providing an adjustable valve flow regulator coupled to the coolant coil at each respective inlet and outlet and the regulating step is performed by at least one of the adjustable valves.
 27. The method of claim 20 further comprising providing a coolant flow bypass between the coolant coil inlet and outlet and selectively recirculating coolant within the coolant coil with the bypass.
 28. The method of claim 27 further comprising providing a remote actuated valve in the bypass for said recirculating within the coolant coil.
 29. The system of claim 12, wherein the coolant coils are replaceable while preserving the original exhaust flue with related connection flanges and the manifolds.
 30. A method for refurbishing the system of claim 12, comprising: disconnecting the coolant coils from the headers; removing the coils from the exhaust flue; installing new coolant coils about the existing exhaust flue; and reconnecting the headers to the new coolant coils.
 31. An exhaust duct cooling system, comprising: an exhaust flue defining a interior cavity for passage of exhaust gas there through along an axial dimension thereof; a coolant coil disposed about the flue for thermal communication with exhaust gas, the coil having a helical profile extending along the flue axial dimension, an interior lumen there through for passage of coolant, and a respective inlet and outlet for respective intake and discharge of coolant.
 32. The system of claim 31, wherein: the coolant coil helical winding profile has an inside diameter of 63-87 inches; the coolant coil has 2-N zone coil circuits; the coils are constructed of pipe selected from the group consisting of 2 inch pipe schedule 80, 2 inch pipe schedule 160, 2.5 inch pipe schedule 80, 2.5 inch pipe schedule 80, 2.5 inch pipe schedule 160, 3 inch pipe schedule 40, and 3 inch pipe schedule 80; and each coil circuit has an internal surface area of 43-76 square feet.
 33. The system of claim 32 wherein the coolant coil has 2-9 zone coil circuits.
 34. The system of claim 5 further comprising a coolant flow bypass between each coolant coil inlet and outlet, for selectively recirculating coolant. 